Engagement of the multicomponent antigen receptor in T cells (TCR) results in rapid activation of a protein tyrosine kinase pathway. A major TCR- associated protein tyrosine kinase is ZAP-70, a protein that binds to the activated TCR. Under conditions of TCR activation, the ZAP-70 bound to the TCR is itself tyrosine phosphorylated and activated. Over the past five years this Section has contributed much biochemical information to further our understanding of the function and regulation of this kinase. Additional analysis of ZAP-70 has been performed in studies in which a chimeric protein comprised of ZAP-70 and the green fluorescent protein has been expressed. With this molecule we have been able to study the dynamic movement of ZAP-70 translocation from cytoplasm to membrane.Our studies also focus on a substrate of tyrosine kinases in T cells, LAT (linker for activation of T cells). This prominent substrate of PTKs activated by TCR engagement is a 36-38kD protein first observed in 1990. We have been able to purify this protein, clone the cDNA that encodes it, and perform a number of studies to characterize how it is phosphorylated and binds a number of critical signaling molecules including phospholipase-Cg, Grb2 and the p85 subunit of phosphoinositide 3-kinase. Binding of these and other proteins is critical to T cell activation. Additional studies demonstrate that LAT is subject to palmitoylation, and via this post-translational addition, is targeted to plasma membrane microdomains.